38 THE MAMMALIA. 



more and more continuous delay in the formation of the teeth is 

 observed ; the teeth eventually no longer cut the gum, and finally 

 disappear completely — a process repeated in many other lines of 

 descent," as in the foetal teeth of many rodents, ungulates, and 

 whales, and called by Professor Cope " the Law of Retardation." 



Four families of CamelidcB survive in South America, but in 

 North America — to all appearance, the original home of the 

 genus — they have become totally extinct. A llama larger than 

 any existing camel survived in North America to Post-Pliocene 

 times. How shall we account for the disappearance of these 

 swarms of the camel tribe from North America, where all condi- 

 tions of existence seemed favourable to them, and how shall we 

 explain their sudden appearance in Asia and continued survival 

 in the Old World ? Possibly, as the camel is now no longer to be 

 found in a wild state, this archaic form has only been preserved 

 through its serviceableness to man. 



It may be interesting to those who are not professed zoologists 

 to explain that the great distinction between the camels and all 

 other living ruminants -is that they have six teeth in addition to the 

 molars in the upper jaw, whereas other ruminants have no incisors 

 or canines whatever in the upper jaw. The teeth in the lower jaw 

 are also less highly specialised than those of oxen or sheep. The 

 feet have two toes encased in small, imperfect hoofs, and the sole 

 of the foot is broad and horny, but the skeleton of the foot is that 

 of a true Ruminant. 



The genus Equus (horse, zebra, and ass) has had an equally 

 curious history of long and slow development, of a palmy time, 

 when it roamed — with numerous species and countless herds — 

 over the plains of North America ; of an extinction apparently as 

 sudden as its rise was slow, an extinction complete and unac- 

 countable. For the American horse survived almost to historical 

 times, and the genus, when re-introduced into America by the 

 Spaniards, throve and multiplied in a wild state, in a manner 

 which showed that continent was still eminently suited to it, both 

 as regarded climate and mode of life. 



The pedigree of the horse is one of peculiar interest, owing 

 to the high state of specialisation reached by this animal, and the 

 completeness with which every step in its progress has been 



